Introduction

Traffic Analysis Zones (TAZs) are used to reduce the spatial complexity of the real world into a useful aggregation for analysis. The aggregate population, employment, and additional metrics contained in each are fed into the model’s various components to determine travel demand.

Most TAZ layers are created by aggregating Census blocks while respecting larger geographies like block groups and tracts.

TAZs

Socio-Economic Data

Population

Reasonableness Checking

Upon receipt of the MPO SE data, SAG performed a number of reasonable checks. The first statistics we checked were:

  • Persons-per-household
  • Vehicles-per-household
  • Workers-per-household

Persons per Household

Overall, the distribution of of persons per household was logical. In the plot below, each zone is plotted based on the number of households and the zonal average persons per household. As expected, zones with many households tend to fall between two and three persons per household.

There is a serious outlier in zone 144, which is Shenandoah University. The population field in an SE data file should represent population in households, not total population. Population in group quarters, such as students in dormitories or correctional inmates need to be modeled separately from the general population.

Workers per Household

The SE data file did not have worker information.

Vehicles per Household

The SE data file did not have vehicle information.

Spatial Checks

In addition to making sure each zone had proper values, a review of the spatial distribution was also conducted to ensure reasonableness. The map below shows the spatial distribution of the following metrics:

  • Household Density (households per sq mi)
  • Employment Density (jobs per sq mi)




Systems Analysis Group, WSP | Parsons Brinckerhoff 2016